The invention relates to a brake booster, in particular, to the construction of a seal unit which maintains an enclosed shell hermetically sealed while supporting a valve body (power piston) for reciprocating motion.
A conventional brake booster is schematically shown in FIG. 5 which illustrates a seal unit for slidably supporting a power piston with respect to a shell. Specifically, a shell 1 includes a cylindrical portion 2 which extends from its rear wall, followed by a portion 3 of a reduced diameter, which defines a step therebetween. The seal unit comprises an annular seal member 4, a bearing 5 and a resilient retainer 6 which are sequentially mounted inside the step. The seal unit 8 slidably supports a rear cylindrical portion 7 of a valve body (power piston) with respect to the shell 1 while maintaining the interior of the shell 1 hermetically sealed against the exterior.
With the conventional arrangement described above, the seal unit requires three components including the seal member 4, bearing 5 and retainer 6. To accommodate for the possibility that one of these components, for example, the bearing 5 may be missed, as a result of an oversight during the mounting operation, there is a proposal as disclosed in Japanese Laid-Open Utility Model Application No. 145,446/1984, for example, in which a metal fixture 9 is bonded to the seal member 4 and is a press fit into a rear shell 12 to thereby secure the seal member 4 in place, the seal member 4 thus functioning both as the bearing and the seal (see FIG. 6). However, in the arrangement of FIG. 6, since the seal member 4 bonded with the fixture 9 is mounted as a press fit into the rear shell 12, a certain precision is required in the size of the rear shell 12, the fixture 9 and the seal member 4 as they are manufactured, presenting a quality control effort or otherwise tending to present a quality which changes from product to product.